During the past 15 years, an invasive plant disease has left a patchwork of dead and dying trees in California’s majestic coastal forests. But the loss of trees is changing more than just panoramic views: The number of ticks that can carry a disease that causes painful joint swelling, fatigue and even neurological damage is growing — a result of the gaps created in the forest when trees die, a recent study found.

To determine how the loss of trees affects ticks, their hosts and the Lyme disease they might carry, researchers at UC Berkeley, UC Santa Barbara and the Cary Institute of Ecosystem Studies in New York have embarked on a multi-year study of animal populations in a North San Francisco Bay forest infected with Sudden Oak Death.

“We know that Sudden Oak Death is infecting and killing a number of tree species all along coastal woodlands in California,” said Andrea Swei, postdoctoral researcher at UC Berkeley and lead author of the study.

Swei said the effects of invasive species are unpredictable, complicated and can have ecological consequences that might at first glance seem totally unrelated, including changes in the risk to humans of contracting certain diseases.

Sudden Oak Death was first reported in 1995 in Marin County and is now found in coastal counties ranging from Monterey to Humboldt, including the forests of Big Sur and the Santa Cruz Mountains. Tanoaks, live oaks and California black oaks are all susceptible to the disease, which is caused by a microbe related to the potato blight that caused the Irish famine of the 1840s.

In addition to being harder hit by the devastating plant disease, humid northern coastal counties are also plagued by the states’ largest risk of Lyme disease, according to Anne Kjemtrup, an epidemiologist with the state Department of Public Health.

Lyme disease is mostly spread by the not-quite-adult — or “nymphal” — stage of the tick, and these poppy seed-sized arachnids fare better in wet areas such as the Northern California Coast. Last year, 7.6 cases of Lyme disease were reported for every 100,000 people in Mendocino County and 6.5 per 100,000 people in Del Norte County, a rate 20 times greater than the state average.

Lyme disease was first detected in the 1970s in Connecticut, and although it is often considered an East Coast affliction, it has been found in every state except Hawaii, according to the U.S. Centers for Disease Control.

The disease has been found in nearly all of California’s 58 counties. In the Forest of Nisene Marks State Park in Aptos, nearly one in five ticks tested positive for the disease in 2000 and 2001, San Jose State University researchers found.

In California, Lyme disease is spread through the bite of the western black-legged tick, which feeds on small rodents, lizards, birds and sometimes deer and other large mammals. But not all of the ticks’ hosts carry the disease, and some can even remove the disease-causing bacteria from the biting pests.

These forest dwellers face a changing environment in the aftermath of Sudden Oak Death, which creates breaks in the forest canopy — a boon to some animals and a threat to others.

The UC study of the relationship between Sudden Oak Death and Lyme disease monitored ticks and four of their animal hosts: deer mice, wood rats, lizards and deer. Researchers found more ticks in forest plots infected with Sudden Oak Death. But whether hikers and rural inhabitants of California’s infected forests are at greater risk of catching Lyme disease was not clear.

Part of the mystery may be the work of an unlikely friend: the western fence lizard.

The reptile is the favorite source of blood meals for nymphal western ticks and its blood has the special ability to kill the bacteria that cause Lyme disease. A tick infected with the disease will leave its lizard dinner free of the microbe — and therefore won’t pass the bacteria onto its next blood source.

When forests become infected with Sudden Oak Death, the “newly created forest gaps result in hotter, drier patches in the forests, which have different implications for different species,” Swei said.

The question at hand is which animal populations grow and which shrink in response to the breaks in the canopy. Lizard populations may do better as temperatures rise from more sun exposure, while nocturnal rodents who live in oaks may lose their homes.

Sudden Oak Death has killed at least one million trees in California, said Katie Palmieri of the California Oak Mortality Task Force, a group that was set up by the California Forest Pest Council and the California Department of Forestry and Fire Protection.

To see how the loss of trees affects animal communities, Swei and her field assistants counted deer droppings, trapped rodents and caught lizards in forests with and without Sudden Oak Death in Marin County. Across all 14 of their experimental plots, they laid 600 pounds of traps, which they had to shift to a new position every week.

“When you walk up to a trap that you set the night before, you don’t know what could be inside,” Swei said. “It could be yet another mouse, or it could be a species you’ve never seen before. It makes it pretty exciting,” Of the tick’s hosts, the researchers found fewer wood rats but more deer mice and western fence lizards in regions affected by Sudden Oak Death.

To count directly whether afflicted forests had more ticks, the research team dragged large flannel cloths across all 14 hectares of their research sites. The researchers stopped at every 50-foot interval of the forest floor and counted the creepy crawlies they had gathered. The team found that tick populations went up as trees died from the disease.

They found that the more oaks a healthy forest had, the fewer infected ticks inhabited it. But they did not find an association between infected ticks and openings in the forest canopy.

However, Swei said, Sudden Oak Death is certainly changing the density of ticks. And because having more oaks in a forest is associated with fewer infected ticks, “there’s a suggestion that Sudden Oak Death could increase Lyme disease risk,” she said.

But because each animal host of the Western tick responds differently to changes caused by Sudden Oak Death — and lizards actually cure the ticks of the disease — the impacts of tree death on Lyme disease risk are not yet clear.

“You have to understand the contribution of each of these vertebrates to the ticks,” said Robert Lane, an expert in tick-borne disease and a co-author of the study. “I think this needs many, many years of study.” Swei will continue to explore how changes in the forest affect Lyme disease ecology. “Because Lyme disease is so complicated, my next step will to be work on mathematical models based on my field data that will help provide a more complete picture,” she said.

Scientists say it’s extremely difficult to predict the ultimate effects of an invasive species like Sudden Oak Death, suspected to have traveled to California on ornamental plants imported from Asia.

“Invasive species are such a problem because the native habitat is na”ve to the invader and can respond in really unexpected ways,” Swei said. “We need to make sure that in this increasingly connected world that we set up ways to block biological stowaways.”

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